1. Field of the Invention
The present invention relates to a mode transformer for a microwave energy transmission circuit.
2. Discussion of the Background
The technique of high power millimetric waves is now being developed due to generators and amplifiers such as gyrotrons, ubitrons, free electron lasers, etc. . .
Microwave energy is transmitted by waveguides. The cross-section of these guides have dimensions which must be chosen while complying with two contradictory restrictions:
a restriction due to the power to be transmitted; PA1 a restriction due to the necessity of propagating the energy in a single mode if possible.
The power to be transmitted imposes dimensions which are sufficiently large to avoid a breakdown due to electrical fields which are too intense; the electrical fields in fact vary in a way which is proportional to the square root of the power and is inversely proportional to the square root of the cross-section of the guide.
The propagation of a single mode in the guide is, on the contrary, ensured when the dimensions of the guide are smaller than a well determined threshold, becoming much smaller as the frequency becomes higher. This threshold is called the cut-off threshold.
These imperatives become contradictory if it is sought to transmit a high power at a high frequency.
There is therefore an obligation to use oversized guides in which several modes can propagate and to impose a single operating mode by a means other than the reduction of the dimensions above the threshold corresponding to the frequency to be transmitted.
Furthermore, microwave energy transmission circuits are generally constituted by devices operating in different electromagnetic modes, for example a generator in mode TE.sub.01, a transmission line in mode TE.sub.02 and an antenna excited in mode TE.sub.11.
In order to connect them a conversion must therefore take place from the output mode of an element to the mode of the following element.
A known solution to the problem of conversion between modes consists in using a waveguide having periodic disturbances in the geometry of the walls, in order to favour the conversion between two modes exhibiting beats having this periodicity along the guide. This solution generally leads to the use of waveguides of long length, for example having a length of several hundred wavelengths.
There is also known, by the application EP-0 171, 149, a mode conversion module using a conical tube whose large end constitutes the input for microwave energy in a first mode and whose small end constitutes the output for microwave energy in a second mode.